1. Field of the Invention
The present invention relates to an oxide semiconductor and a semiconductor device including a transistor using the oxide semiconductor in a channel region.
2. Description of the Related Art
An oxide semiconductor has attracted attention as a material for transistors. The oxide semiconductor is said to have a feature similar to that of amorphous silicon capable of being formed at a low temperature, and a feature similar to that of polycrystalline silicon having high field-effect mobility. For example, the following is disclosed: a field-effect transistor using an amorphous oxide, whose electron carrier concentration is controlled, and an image display device using the transistor (Patent Document 1).
The transistor using an amorphous oxide semiconductor formed at room temperature, however, has a problem in characteristics: not enough high field-effect mobility, low reliability, and large hysteresis. In order to solve such a problem, a technique of crystallizing an oxide semiconductor by laser irradiation is disclosed (Patent Documents 2 and 3). These documents show that laser irradiation performed on an oxide semiconductor film allows crystallization of an amorphous oxide semiconductor.